1kg = 1000grams
mass of uranium = 235
number of atoms in 1 mole = avogadro's number = 6.02 x 10²³
Each nucleus will release = 200 Mev of energy.
So, E = 200 x (1000/235) x 6.02 x 10²³
= 5.12 x 10²⁶ Mev
Now convert this to joules
1Mev = 1.60218 x 10⁻¹³
5.12 x 10²⁶ Mev = 5.12 x 10²⁶ Mev x 1.60218 x 10⁻¹³
= 8.21 x 10¹³ Joules